There are numerous radio-frequency anti-theft and signaling security devices that generate an alarm when an object or person exceeds a prescribed distance, fixed or adjustable, between a transmitter and receiver.
In some cases, a coded signal is transmitted in order to differentiate between otherwise identical units located in a particular area.
All of these existing systems consists of a signal transmitter and receiver transmitting a signal with or without an identifying code. The transmitter sends out an r.f. signal which is received by the receiver. The receiver amplifies the incoming signal and triggers an alarm when the level of the signal falls below a prescribed value. If it is desired to set a range beyond which an alarm will be generated, an adjustable dial presets the sensitivity level of the alarm trigger. One fundamental problem, however, is common to all these systems. They depend wholly on r.f. signal strength for their ranging capabilities.
The use of signal strength as a range measurement can be seriously compromised by many factors. For example, placement of the transmitter and receiving antennas relative to each other can severely attenuate signal strength depending on the r.f. environment local to each thereby rendering an inaccurately triggered alarm. The mere bodily rotation of a person whose transmitter is at one point, even in a line of sight, to the receiver will result in a drastic change in signal strength if that person's body is rotated 180 degrees causing the body to lie between the antennas. In addition, hills or dips in the terrain, r.f. reflectors such as buildings, autos, trees, and wet or dry conditions will all severly affect the received signal strength.
R.f. levels at a given range can vary up to 30 dB or more, depending on the above mentioned circumstances which is equivalent to a change in a range reading of up to 500% or more.
Tests have indicated that changes in range measurements from 2 to 1 to 10 to 1 can be obtained depending on the r.f. frequency used for the transmissions which determines whether field strength varies as 1/R or 1/R squared (where R=Range) and how perfectly omnidirectional the antenna design is, as well as the above factors.
There are other methods that have been devised for ranging an object and have been in use for a considerable length of time. One method involves the transmission of a high powered r.f. pulse which is reflected off the object to be ranged and received by the receiver where the time delay is translated into distance. This principle works well provided there is a sufficient power source but has the disadvantage that the passive reflector causes attenuation of the transmitted signal power by 1/R to the fourth power. The method therefore requires a very high power level coupled to a very sensitive receiver.
Another method used for ranging purposes is by use of a laser beam. In this case, a short high powered light pulse is directed towards the target which then reflects the pulse back to the source where the time delay is measured and translated into distance. Somewhat more efficient, the power attenuation of the returned signal is 1/R squared power. The disadvantage however, is that one needs to seek and locate the object or individual so that it may be targeted.
The present invention incorporates an omnidirectional antenna with no preferred transmission direction required, since the control unit and corresponding transponder identify each other by other means (e.g. digital coding, r.f. frequency band pair, modulating frequencies, etc.)